Fluorocarbons are liquids that hold large amounts of oxygen and give it off to the tissues. Thus they have great potential as substitutes for whole blood. Long term survival has been observed after exposure to massive amounts of fluorocarbon emulsion given intravascularly. In this application we propose to study many aspects of cerebral microcirculation during periods of blood replacement with fluorocarbon. Observations will be made of transit time, red cell velocity, aggregation of residual red cells, vascular permeability, pharmacologic responses, and responses to superimposed anoxic or hemodynamic stress. Measurements of blood viscosity and of brain O2 tension, NADH level, and EEG will be made, and changes in these variables will be related to the microcirculatory variables mentioned above. In addition, light and electron microscopic observations will be made of brain, lung, liver, spleen, kidney and muscle, in an effort to detect toxic effects of the fluorocarbon emulsion, in mice, Rhesus monkeys, and other animals, along with analysis of survival time and physiologic variables such as blood gas tensions.